Generally, an image sensor is a semiconductor device that converts an optical image to an electric signal. An image sensor is typically classified into a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor.
The CMOS image sensor includes a photodiode and a metal oxide semiconductor (MOS) transistor in a unit pixel, and sequentially detects electrical signals of unit pixels in a switching manner to realize an image.
FIG. 1 is a photograph view of a punch-through of a CMOS image sensor according to the prior art.
In the CMOS image sensor according to the prior art, a wide depletion region is formed due to the low impurity doping concentration of a N−-type diffusion region of a photodiode and the low impurity doping concentration of an epitaxial layer formed on a semiconductor substrate.
FIG. 1 shows a punch-through ‘A’ occurring. The punch-through ‘A’ is a phenomenon where a photodiode region 69 contacts a floating diffusion region 72 due to the wide depletion area of the photodiode formed between an N−-type diffusion region of the photodiode 69 and an N+-type floating diffusion region 72, which is the drain region of a transistor 32. When the punch-through ‘A’ occurs, a dark current increases, which decreases characteristics of the photodiode.
Also, the CMOS image sensor according to the prior art generates noise because of the increase of dark current.